Nondeterministic ball bounce[SOLVED]

I am getting different behaviors with exactly the same execution and world settings. As you can see in this video , I reset the world every time and launch the ball with exactly the same velocity. This happens when launching the ball against any kind of surface (not only this board).

This is a pretty lame answer, but--numerical instability.
My guess is that this is an issue with the underlying computations done in gazebo, and more importantly in whichever physics engine you're using. It's also possible the ball is moving slightly around the launcher, and the timing of when you fire matters.